(1) Field of the Invention
The present invention relates to a method for making a semiconductor substrate having a strained silicon layer for improved semiconductor device performance. More particularly the method utilizes a porous silicon layer and a relaxed silicon germanium (SiGe) layer to achieve a strained Si layer for improved device performance (mobility). One application of this strained layer is to improve the channel mobility for FETs in CMOS applications on Silicon-On-Insulator (SOI) substrates (also referred to as wafers).
(2) Description of the Prior Art
In past years silicon semiconductor devices such as bipolar transistors, FETs were reduced in size to improve device performance. The devices were scaled in both the vertical direction and horizontal direction using improved photolithographic and directional plasma etching techniques and self-aligning techniques. As the minimum feature sizes of the devices decrease, the spacings between the metal lines used to wire up the devices decrease and there is a corresponding increase in pitch. This closer spacing results in increased RC time constants. To achieve further improvements in performance, it is necessary to utilize low-dielectric insulators and higher conducting metal-lurgies, such as copper, to reduce the RC time constant of the circuit. Further increase in performance is achieved by using a Silicon-On-Insulator to minimize the capacitive coupling of the semiconductor devices to the substrate and also to improve the radiation hardening for the devices.
As the minimum feature sizes and wiring pitch approach their limits, it is necessary to explore other methods to improve device performance. One approach is to form strained silicon layers on silicon germanium semiconductors to modify the silicon band structure resulting in higher electron and hole mobility.
Several methods of fabricating wafers using SiGe to increase current mobility have been reported in the literature. For example, Ek et al., U.S. Pat. No. 5,759,898, show a method for making a substrate in which a thin silicon layer is used as a buffer layer for forming a relaxed SiGe layer having reduced crystalline defects. Then a strained silicon layer is formed on the relaxed SiGe layer. Devices having higher electron mobility are formed in the strained silicon layer. U.S. Pat. No. 6,107,653 to Fitzgerald shows a method of forming a Go layer with minimal defects on a  less than 001 greater than  silicon substrate using a series of graded GeSi layers having planarization steps in between. The grading of the GeSi layers and the planarization control dislocation densities. This allows a 100% Ge layer to be formed on a Si substrate without an increase in threading dislocation densities. Sato, U.S. Pat. No. 6,143,629, teaches a process for making a smooth single-crystal layer on the surface of a porous silicon layer, which allows Sato to grow an epitaxial Si layer, which results in fewer stacking faults. Sato does not address forming a relaxed SiGe layer or a strained Si layer. Sakaguchi et al., U.S. Pat. No. 6,294,478 B1, teach a process for forming a silicon-on-insulator substrate. The method utilizes a process in which a first Si substrate has a porous Si layer. A non-porous thin Si film is formed through the porous Si layer, and a second substrate is bonded to the first substrate. The porous Si is removed to separate the two substrates.
There is still a need in the semiconductor industry to improve upon the process for making strained silicon layers on a silicon germanium film for silicon-on-insulator substrates to increase electron and hole mobility, and using a cost-effective manufacturing process.
A principal object of this invention is to provide a strained silicon layer having increased electron and hole mobility on a silicon-on-insulator substrate for making semiconductor devices.
A second object of the present invention is to use a sacrificial layer that is mechanically weak, such as a porous silicon layer, with an overlying relaxed SiGe layer to form the strained silicon layer on a seed wafer.
A further object of this invention is to bond a handle wafer to the seed wafer, and to etch and separate the wafers at the sacrificial layer to expose the strained Si layer.
In summary, by a first embodiment, the objects of the present invention for making a strained silicon layer on a SOI wafer (handle wafer) begins by providing a polished silicon wafer (seed wafer) having an atomically smooth principal surface. A sacrificial layer, such as a porous silicon layer, is formed on and in the principal surface of the seed wafer, for example, by local anodization. The sacrificial layer is processed to form a smooth surface. If
the mechanically weak layer is porous silicon, the smooth surface may be formed by high temperature (for example,  greater than 1000xc2x0 C.) annealing in hydrogen ambient. A relaxed silicon germanium layer is formed on the smoothed surface of the sacrificial layer, for example, by growing an epitaxial layer. Next, an epitaxial silicon layer is grown on the SiGe layer to provide a bonding surface for bonding the seed wafer to a handle wafer. Then, a handle wafer having an insulator on a principal surface is bonded to the epitaxial silicon layer on the seed wafer. The sacrificial layer is etched to separate the seed wafer from the handle wafer and to expose portions of the sacrificial layer and the smoothed silicon layer on the surface of the relaxed SiGe layer, which is now on the insulator of the handle wafer. A second anneal is used to anneal the porous Si layer on the handle wafer and to form a smooth strained Si layer on the underlying SiGe layer. This completes the silicon-on insulator having the strained silicon layer on a principal surface.
A second embodiment of this invention is similar to the first embodiment. The difference is that a silicon oxide (SiO2) layer is formed on the relaxed SiGe surface and is used instead of a Si epitaxial layer as the bonding layer on the seed wafer.
A third embodiment of this invention is similar to the first embodiment. The difference is that a SiO2 layer is formed on the Si epitaxial layer on the relaxed SiGe surface to provide a bonding layer on the seed wafer.